Large, powerful and safe lithium-based energy storage devices represent an essential component in developing alternative drive concepts, e.g. for hybrid vehicles or renewable energy techniques, e.g. for storing the electricity produced by wind power. Lithium ion batteries, or LIB, are now the most widely used power sources for mobile applications. The electrolyte between the electrodes comprises dissolved lithium salt. We refer to lithium ion accumulators, lithium polymer accumulators, or lithium solid-state accumulators according to whether the electrolyte is liquid or solid.
In this context, the development of solid-state electrolytes may be the key to the next generation of lithium batteries. When using metallic lithium as an anode, they have a considerably higher energy density and are advantageously less flammable, since it is no longer necessary to use organic components in the battery components. Due to their usually gel-type or ceramic electrolytes, they also have better flow properties and can be used at higher temperatures due to their wide stability range.
Substituted lithium titanium phosphates have already been extensively researched as potential candidates for a solid-state electrolyte conducting lithium ions and assessed as being very promising due to their high ion conductivity and mechanical stability.
Lithium titanium phosphates' potential as an electrolyte has been known for quite some time. Lithium titanium phosphate crystallizes in the so-called NASICON structure. NASICON stands for “Sodium (NA) Super Ionic CONductor”, and refers to a group of solids having the chemical formula Na1+xZr2SixP3−xO12 where 0<x<3.
NASICON is also used to refer to similar compounds in which Na, Zr and/or Si can be replaced by isovalent elements and crystallize in the same structure.
NASICON compounds are generally characterized by high ionic conductivity ranging from 10−5 to 10−3 S/cm at room temperature. At higher temperatures of 100-300° C., ionic conductivity increases to 10−2-10−1 S/cm and is thus comparable with liquid electrolytes. This high conductivity is caused by the mobility of the Na or Li ions within the NASICON crystal lattice.
The crystal structure of NASICON compounds consists of a covalent network of ZrO6 octahedrons and PO4/SiO4 tetrahedrons joined via shared edges. The Na or Li ions are located on two different interstices, between which they are able to move. In this process they have to pass through so-called bottlenecks. The size of the bottlenecks influences ionic conductivity due to steric interaction of the Na ions with the local environment of the Zr2(P,Si)3O12 lattice and is dependent on the specific composition of the NASICON compound and the acid content of the surrounding atmosphere. Ionic conductivity can be increased by adding a rare earth element such as yttrium, for example, to the NASICON compound.
Partial substitution of Ti4+ cations by a trivalent M3+ cation such as Al3+, Y3+ or Sc3+ may cause a defect in the positive charge in the crystal structure of lithium titanium phosphates, which can be compensated by additional Li+ ions, leading to higher ionic conductivity overall as the number of charge carriers is increased as a result.
The ionic conductivity figures for substituted LTP materials published in literature to date are typically in the region of 1×10−4 to 1×10−3 S/cm, and are thus the highest values known for solid oxide electrolytes in literature, alongside garnets of the Li7La2Zr3O12 type which conduct Li ions. It was assumed that a further substitution of phosphate groups by silicate groups according to the general formula Li1+x+yMxTi2−x(PO4)3−y(SiO4)y would increase ionic conductivity and mechanical stability still further (U.S. Pat. No. 6,475,677 B1).
A number of different processes for preparing LTP-based powders are already known in the art. These include the solid phase reaction, the sol-gel method, and the melt-quenching technique (melting followed by quenching).
However, one of the major challenges when preparing LTP-based materials is ensuring phase purity of the prepared powders. In the traditional process using a solid phase reaction, the prepared powders generally contain internal impurities. These disadvantageously lead to a reduction in the ionic conductivity of these powders.
A further problem when preparing LTP-based materials is the compaction required for many applications, as the temperature required during compaction is usually very close to the decomposition temperature of these materials. To date, the only methods capable of resolving the above-mentioned problems have been expensive and complex.
U.S. Pat. No. 6,475,677 B1 describes the method used in the melt-quenching process by way of example. In this case the source material (stoichiometric quantities of NH4H2PO4, AI(PO3)3, LiCO3, SiO2 and TiO2) is initially melted at approximately 1500° C. and then cooled in a water bath so that it recrystallizes at 950° C. to prepare Li1+x+yMxTi2−x(PO4)3−y(SiO4)y where 0≤x≤0.4 and 0≤y≤0.6. The resulting glass ceramic was milled in a ball mill until average particle sizes of 7 μm were obtained.
Wen et al, “Preparation, Microstructure and Electrical Properties of Li1.4Al0.4Ti1.6(PO4)3 Nanoceramics,” J. Electroceram, Volume 22, 2009, Pages 342-345, report that almost 100% of the theoretical density is achieved by compacting prepared Li1.4Al0.4Ti1.6(PO4)3 powder using a sol-gel method by spark laser sintering. This led to an Li ion conductivity of 1.39×10−3 S/cm at room temperature. However, the laser device used for this purpose seems to be unsuitable for industrial production.
M. Holzapfel et al (US 2012/0295168 A1) describe a spray drying method by means of which Li1+xAlxTi2−x(PO4)3 where x≤0.4 was prepared as “phase-pure” powder in the first instance. In this process, corresponding quantities of lithium, aluminum and titanium salts or corresponding oxides were first dissolved in phosphoric acid. The primary powders were then obtained by spray drying the solution. The “phase-pure” Li1+xAlxTi2−x(PO4)3 powder could then be prepared by sintering the primary powders at approximately 900° C. “Phase-pure” is understood to mean that foreign phases, such as AIPO4 or TiP2O7 are present in quantities of less than 1% of the total. The document does not disclose tests on the density of the Li1+xAlxTi2−x(PO4)3 green bodies. There is also the question of the hazards posed by spray drying phosphoric acid-based solutions.
DE 10 2012 103 409 B3 also describes another method for preparing Li1+x+yAlxTi2−xP3−ySiyO12 powder where x≤0.4. This document discloses a sol-gel method for preparation purposes. Aqueous lithium and aluminum salt solutions are mixed with a titanium alkoxide and, if applicable, an orthosilicate in corresponding proportions to form a sol. The aqueous solutions have a pH value in the neutral to alkaline range from 7 to 12. An aqueous ammonium dihydrogen phosphate solution is then added to the sol, causing a gel to form. The gel is formed as a result of a condensation and polymerization reaction initiated by combining the alkoxide and phosphate solutions without the need for additional auxiliary substances such as glycol or citric acid.
The final pure powder is then obtained by drying and calcining the gel. The resulting average particle sizes range from 0.5 μm to 5 μm. All process steps prior to calcining take place at room temperature. The heat treatment should ensure pyrolytic decomposition of disruptive elements which evaporate off.
However, DE 10 2012 103 409 B3 explains that impurities in the Li1+x+yAlxTi2−xP3−ySiyO12 powder cannot be completely ruled out in the synthesis method thus described. It reports that, despite the foreign phases that occur, a lithium ion conductivity in the region of 1×10−3 S/cm would still be achieved at room temperature. Furthermore, a dense sintered product would be obtained by sintering at a pressure of between 5 and 50 MPa.